So...after complete warm-up with fan off for five minutes...when you turned fan on high...did you get good heat for about 5-10 seconds before the heat faded to mediocre? No = blend air door problem. Yes = heater core is plugged, or flow is restricted, or persistent air pocket, or water pump is doing a poor job at low rpm. Does heater output temp improve a lot if you hold RPM about 3000 for 30 seconds?
I wouldn't think that the water pump had an issue since it is cooling the engine as expected, never runs hot or over heats. The whole time I was checking the temps this morning was idling in the driveway.